1. Field of the Invention
The present invention relates to a thermal field emission cathode which is used as an electron beam source for e.g. an electron microscope, an electron beam lithography system, an electron IC beam tester or a wafer inspection equipment.
2. Discussion of Background
In recent years, in order to obtain an electron beam with higher brightness, a thermal field emission cathode employing a needle electrode of tungsten single crystal has been utilized. This thermal field emission cathode is one having a coating layer of zirconium and oxygen (hereinafter referred to as the ZrO coating layer) formed on a tungsten single crystal chip (hereinafter referred to as the W-chip) having an axis direction of &lt;100&gt;, so that the work function of (100) face of the tungsten single crystal is reduced to about 2.8 eV by the ZrO coating layer, whereby only the very fine crystal facet corresponding to the (100) face formed at the forward end of the W-chip, constitutes an electron emitting region, and whereby an electron beam with higher brightness than conventional thermoionic cathodes, can be obtained, and yet, it has a characteristic of long service life. Further, it has more stable electroemitting characteristics than a cold field emission cathode has, so that it can be operated under a relaxed vacuum degree and is easy to use.
As shown in FIG. 1, the thermal field emission cathode comprises the W-chip 1 for emitting an electron beam, which is fixed by e.g. welding to a predetermined position of a tungsten wire 3 supported by metal supports 5 fixed to an insulating glass 4, and a suppressor electrode 2 for forming an electric field to suppress thermoionic emission from e.g. the above mentioned tungsten wire 3.
As shown in FIG. 2, a source for supplying zirconium and oxide, i.e. a reservoir 6, is provided at a portion of the W-chip 1. Although not shown in the Figure, the surface of the W-chip 1 is covered by a ZrO coating layer. The W-chip 1 is heated by electric current through the tungsten wire 3 and is used at a temperature of about 1800 K, whereby the ZrO coating layer on the surface of the W-chip 1 evaporates. However, zirconium and oxygen will diffuse from the reservoir 6 and will be continuously supplied to the surface of the W-chip 1, and consequently, the ZrO coating layer will be maintained.
A method comprising the following three steps, is known as a conventional method for accomplishing a low work function by forming a ZrO coating layer on the W-chip.
First step: A solvent such as an organic solvent, is added to a powder of zirconium hydride as a zirconium-containing material, to obtain a slurry, which is then attached to the W-chip having an axis direction of &lt;100&gt;, to form a lump of zirconium hydride.
Second step: The W-chip is heated under high vacuum to decompose zirconium hydride into zirconium and hydrogen, thereby to diffuse zirconium into the W-chip surface.
Third step: The W-chip is heated in an oxygen atmosphere of about 10.sup.-6 Torr to form a ZrO coating layer on the W-chip surface. (see U.S. Pat. No. 4,324,999.)
Such a conventional thermal field emission cathode has had a problem that the frequent temperature rise and drop under a practical operation causes cracks in the reservoir, and in an extreme case, the reservoir falls off, whereby the service life of the thermal field emission cathode tends to be substantially short. Therefore, in practical use of such a thermal field emission cathode, it has been common to set a restriction in its use to avoid frequent temperature rise and drop and to maintain the operation temperature constant without raising or lowering the temperature as far as possible once the operation temperature has been set.
However, it is unavoidable to repeat raising and lowering the temperature of the thermal field emission cathode during the production and adjustment of an electron beam equipment. Likewise, it is unavoidable to raise and lower the temperature many times for the maintenance of the equipment also in the practical operation. Further, due to an unexpected trouble, the temperature may instantaneously drop. Accordingly, it has been desired to develop a thermal field emission cathode free from falling off of the reservoir.